This invention relates generally to broadband communications systems, such as cable television systems and the optical equipment used in such systems, and more specifically to the transmission of optical signals in a broadband communication system.
Communication systems, such as a cable television system, include a headend section for receiving satellite signals and demodulating the signals to baseband. The baseband signal is then converted to an optical signal for transmission from the headend section over fiber optic cable. Optical transmitters are utilized within the cable system for imprinting radio frequency (RF) signal information onto an optical beam, where the optical signal is then split and applied to various optical fibers within the transmitter for transmission to remotely located optical receivers. The optical receivers are provided for receiving the optical signals and converting the optical signals back to RF signals, which are further transmitted along branches of the system over coaxial cable rather than fiber optic cable. Taps are situated along the coaxial cable to tap off a portion of the signal to subscribers of the system.
A primary aim of communication systems is to provide the highest quality signal to each subscriber on the network. It is, therefore, important to understand the parameters that affect network planning and implementation. Prominent among these parameters are amplitudes of the optical signal carriers, i.e., carrier level, and noise. It is well known that noise exists in conventional electrical systems, as well as optical systems, in which noise is further classified as thermal, shot, and relative intensity noise (RIN), the latter being exclusive to optical transmission systems. Carrier-to-noise ratio (CNR) is a useful measurement of the effects of noise upon signals. A goal of most high quality services delivery networks is to maintain the ratio of carrier signal level to noise as high as possible. The maintenance of a high CNR becomes more important as networks grow larger and implement two-way services.
Although employing simplistic techniques, such as decreasing the fiber lengths between optical nodes, can mitigate CNR problems, such techniques decrease the number of users served by the network, decrease the network""s physical reach, decrease the value of the network to its users, and ultimately, increase costs to both subscribers and service providers. Thus, any solution to the problem of degradation of CNR in large system networks should offer a better way to provide reliable and accurate transmission of optical signals within a cable television system while maintaining or increasing all existing functionality.